Messaging systems are well-known in the art. One of the simplest form of messaging system is a voice messaging system. Computer Telephony Integration (CTI) is a term which refers to the integration of computer architectures with telephony systems. A voice messaging system is an example of a CTI system, and a further example is a unified messaging system.
Voice or unified messaging systems have conventionally been deployed in one of three implementations.
In a first voice messaging implementation a voice messaging (or unified messaging) system is co-located with a subscriber's telephone switch, i.e. a subscriber's private branch exchange (PBX). Within a multi-site organization or enterprise this will tend to result in each physical location (which has its own telephone switch) having a separate voice messaging (or unified messaging) system. For an enterprise with many distributed locations, such an arrangement requires the deployment of many (possibly small) systems resulting in considerable IT administrative costs.
FIG. 1 shows two distributed locations each with their own telephone switch, specifically a PBX 102 and 106 respectively, and respective dedicated unified or voice messaging systems 104 and 108. This system provides, at least, the following three typical voice messaging functions: call answering, automated attendant and subscriber access.    1. The call answering feature is initiated when calls are forwarded by the telephone system as a result of the called party not answering (e.g. due to a busy or no-answer condition).    2. The automated attendant feature offers callers a menu for automatically routing a call to the desired answering point, without the need for operator intervention.    3. The subscriber access feature allows subscribers, or mailbox owners, of the voice messaging system to call into the system and retrieve their messages over the phone.
In a second voice messaging implementation, the voice messaging system utilizes a deployment of a central messaging system serving multiple remote locations. This form of deployment requires long-distance telephone connections to be provided between the remote private branch exchanges (PBXs), serving remote locations, and the centralized messaging system.
FIG. 2 shows two locations each having a respective PBX 202 and 204 being served by a single centralized unified or voice messaging system 206 associated with a single centralized PBX 208. The long distance telephone connections generally need to be provided regardless of their use (i.e. it is necessary to purchase enough capacity to handle busy periods). In addition some voice messaging features are lost in the centralized arrangement of FIG. 2. The automated attendant application provides callers into an office or building with the ability to connect to a user. This capability is not provided in the centralized arrangement of FIG. 2: the system providing the automated attendant function handles all users for all systems, and so does not provide service equivalent to a human attendant at the remote site. Additionally, different offices may be located in different countries with different language requirements. A simple example is that a first office may be in Canada and need both English and French languages, whereas another office served from the same centralized facility may have entirely, non-overlapping, language requirements, for example located in Mexico and requiring Spanish. A common and unique numbering plan across all mailboxes across all sites is also needed.
In a third implementation the second implementation is modified such that the long-distance telephone connectivity is provided by an organisation's (existing) data (IP) Wide Area network (WAN), possibly using Voice over Internet Protocol (VoIP). This deployment puts a strict requirement on the bandwidth and more critically the quality of service (e.g. the network latency) characteristics of the corporation's WAN. This most generally is not, and will not, be met for the majority of corporations now or in the near future.
It is an aim of the present invention to provide an improved messaging system. A further aim is to provide such a system offering increased deployment flexibility.